What is the total tonnage minus exempted spaces referred to as?

The term that refers to the total tonnage of a vessel minus exempted spaces is net tonnage. Net tonnage is a measurement that represents the volume of a ship's cargo-carrying capacity after accounting for spaces that are not used for cargo, such as crew accommodations, engine room, and other spaces that do not contribute to the vessel's revenue-generating capabilities. This measure is significant for determining port fees and other regulations since it reflects the revenue-earning potential of the ship.

In contrast, gross tonnage measures the overall internal volume of a ship, including all enclosed spaces, while deadweight tonnage refers to the total weight a vessel can carry, including cargo, fuel, and supplies. Lightship tonnage indicates the weight of the ship itself without any cargo, fuel, or stores on board. Understanding these distinctions is key to properly assessing a vessel's capacity and the implications for operational and regulatory contexts.